Although many castings are referred to by specific characteristics such as airflow capacity or choke type, Rochester actually gave each Quadrajet style different designations indicating body type and choke style. The first design series was designated 4M and can be found on all GM vehicles through 1974 and on GMC and Chevy trucks through 1977. When Rochester redesigned the Quadrajet in 1975, they referred to it in factory literature as the “Modified Quadrajet” or “Mod-Quad,” and designated it M4M. When computer-operated electronic internal components were added around 1980, Rochester designated the Electronic Quadrajet the E4M.

The last letter in the series designation generally indicates choke type.

The letter “V” indicates an automatic choke where the thermostatic choke coil is fastened to the exhaust crossover on the intake manifold. Commonly referred to as a “divorced” choke, a small rod connects the coil to the mechanical linkage on the passenger side of the main body.

The letter “C” indicates a carburetor-mounted automatic choke. In these instances, the thermostatic coil, which is mounted on the passenger side of the main body, uses heated vacuum and is easily recognizable by its large, black adjust­ment disc. They are generally referred to as a “hot air” style. Most carburetors produced in and after about 1975 have the choke housing mounted on the carburetor. The thermo­static coil is activated by hot air from the intake manifold exhaust crossover system via a heated tube. Later, electric chokes started showing up in certain applications to replace the hot-air-style chokes.

The letter “E” indicates an electronic choke coil. They too, are recognizable by a large black disc, but they operate off of a 12-volt source as opposed to vacuum. And those Quadrajets without any letters following the 4M andM4M designation had a manual choke.

The list below identifies the different types of Quadrajet designations that appear in AC Delco printed information.

4M: Quadrajet with manual choke

4MC: Quadrajet with carburetor-mounted automaticchoke

4MV: Quadrajet with divorced automatic choke

4ME: Quadrajet with carburetor-mounted electric choke

M4MC: “Modified” Quadrajet with carburetor-mounted automatic choke

M4ME: “Modified” Quadrajet with carburetor-mounted electric choke

4MEA: “Modified” Quadrajet with carburetor-mounted electric choke and “Altitude” calibration

E4MC: “Electronic” Quadrajet with carburetor-mounted automatic choke

E4ME: “Electronic” Quadrajet with carburetor-mounted electric choke


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